Hitherto, a main material used in an automobile industry has been steel. However, movements associated with energy saving have been stepped up all across the world, and there is a tendency of demand for further weight reduction of vehicles. The weight reduction is planned not only for a vehicle itself but also for the parts constituting thereof and fastening parts including bolts.
As regards steel bolts, the size thereof has been reduced by enhancing the strength, and the weight reduction has been promoted through a means of reducing the number of pieces involved; however, even though the number of pieces involved is suppressed to a required minimum, further weight reduction has been difficult to be realized since the specific gravity of the material itself is large.
Moreover, in a trend of rendering constituent parts of a vehicle to be of aluminum, usage in combination with steel-made bolts brings about fears of a cave-in of an aluminum material due to the difference in thermal expansion coefficients and of development of corrosive deterioration due to contact of dissimilar metals, and also brings about a problem of requiring the number of man-hours for separation at the recycling stage. Accordingly, an aluminum based alloy-made bolt (hereinafter, referred to as “aluminum alloy bolt”) having high strength is demanded.
In the case of making an aluminum alloy bolt highly strengthened, it has hitherto been common to condition it to have predetermined strength through implementation of a solution treatment and an ageing treatment as described in Patent Literature 1. Since the aluminum alloy bolt is apt to suffer a crack due to stress corrosion with enhancing in strength, overage thermal refining by which the strength is lowered by 5 to 15% from the maximum strength is generally utilized.
The method described in Patent Literature 1 is to apply a heat treatment, which includes quenching from a temperature exceeding 480° C. and an ageing treatment within the range between +60° C. and 220° C., after being formed into a bolt shape through cold heading.
Likewise, as regards Patent Literatures 2 to 4, a heat treatment for enhancing strength is to be applied after a shaping process as with Patent Literature 1.